1000 TIN 



the yellow sulphide of copper. A great quantity of tin has been produced by ' stream- 

 ing' (as washing the debris in the valleys is termed); and this variety, called ' stream- 

 tin/ produces the highest price in the market. Very little stream-tin is now obtained. 



The Cornish ores occur 1, in small strata or veins, or in masses; 2, in congeries 

 of small veins ; 3, in large veins ; and 4, disseminated in alluvial deposits, as described. 



The stanniferous small veins, or thin flat masses, though of small extent, are some- 

 times very numerous, interposed between certain rocks, parallel to their beds, and are 

 commonly called tin-floors. In the mine of Bottalack a tin-floor has been found in the 

 killas (a schistose rock), thirty-six fathoms below the level of the sea ; it is about a 

 foot and a half thick, and occupies the space between a principal vein and its ramifi- 

 cation ; but there seems to be no connection between the floor and the great vein. 



2. StockwerJcs, as the Germans term the disseminated masses, occur in granite and 

 in the felspar porphyry, called in Cornwall elvan. The most remarkable of these, in 

 the granite, is at the tin-mine of Carclase, near St. Austell. The works are carried 

 on in the open air, in a friable granite, containing felspar kaolin, or china-clay, 

 which is traversed by a great many small veins, composed of tourmaline, quartz, and 

 a little tin-stone, that form black delineations on the face of the light-grey granite. 

 The thickness of these little veins rarely exceeds 6 inches, including the adhering 

 solidified granite, and is occasionally much less. Some of them run nearly east and 

 west, with an almost vertical dip ; others, with the same direction, incline to the south 

 at an angle with the horizon of 70 degrees. 



Stanniferous masses are much more frequent in the elvan (porphyry) ; of which 

 the mine of Trewidden is a remarkable example. It was worked among flattened 

 masses of elvan, separated by strata of killas, which dip to the east-north-east at a 

 considerable angle. The tin ore occurred in small veins, varying in thickness from 

 half an inch to 8 or 9 inches, which were irregular, and so much interrupted that it 

 was difficult to determine either their direction or their inclination. 



3. The large and proper metalliferous veins are not equally distributed over the 

 surface of Cornwall and the adjoining part of Devonshire ; but are grouped into three 

 districts: namely, 1, In the south-west of Cornwall, beyond Truro; 2, In the neigh- 

 bourhood of St. Austell ; and 3, In the neighbourhood of Dartmoor in Devonshire. 



The first group is by far the richest and the best explored. The great tin-veins 

 are the most ancient metalliferous veins in Cornwall ; yet they are not all of one 

 formation, but belong to two or more different systems. Their direction is, however, 

 nearly the same, but some of them dip towards tho north, and others towards the 

 south. It was formerly thought by the Cornish miners that tin occurred in the upper 

 portions of the mineral lodes only, and mines were abandoned, when in sinking the 

 miners came to the ' yellows ' copper pyrites, which were said ' to have cut out the 

 tin.' Within the last few years, however, tin has been found at very great depths 

 below the surface and beneath the copper. Dolcoath Mine is a very remarkable 

 example of this. This mine was first worked as a tin mine for a very long period ; 

 then as a copper mine for half a century ; and then, upon persevering in depth, the lode 

 was found to become more and more rich in tin, which is now worked to great advan- 

 tage. Other mines in the same locality have presented similar conditions. 



At Trevaunance Mine the two systems of tin-veins are, both, intersected by the 

 oldest of the copper- veins ; indicating the prior existence of the tin-veins. In flg. 



1981, b marks the first system of tin-veins; c the 

 second; and d the east and west copper-veins. 

 Some of these tin-veins, as at Poldice, have been 

 traced over an extent of two miles ; and they vary 

 in thickness from a small fraction of an inch to 

 several feet, the average width being from 2 to 4 

 feet; though this does not continue uniform for 

 any length, as these veins are subject to con- 

 tinual narrowings and expansions. The gangue 

 is quartz, chlorite, tourmaline, and sometimes 

 decomposed granite and fluor-spar. 



4. Alluvial tin ore, Stream-tin. Peroxide of tin occurs disseminated both in tho 

 alluvium which covers the gentle slopes of the hills adjoining the rich tin mines, and 

 also in the alluvium which fills the valleys that wind round their base ; and in these 

 deposits the tin-stone has been so abundant that for centuries the whole of the tin of 

 Cornwall was derived from them ; and it is still so to some extent. The most 

 important explorations of alluvial tin ore are grouped in the environs of St. Just and 

 St. Austell, where they are called stream-works, because water is the principal agent 

 employed to separate the metallic oxide from the sand and gravel. 



The most extensive and productive stream-works were formerly those of Pentewan, 

 near St. Austell, 



